1. Field of the Invention
The invention concerns a device for determining the concentration of solid particles in a particle-loaded gas stream with a pre-diluter mixing a sample flow branched off from the gas stream with a substantially particle-free diluting gas, a heated evaporator downstream in the diluted sample flow, a secondary diluter downstream of the evaporator and mixing the sample flow with additional substantially particle-free diluting gas, and a particle counter downstream of the secondary diluter.
2. The Prior Art
Devices of the type mentioned and the processes performed with such are known particularly in connection with the characterization and measurements of aerosols in the exhaust gas of combustion engines and are at least partially also already the subject matter of national as well as regional and international test regulations, standards and the like. A relatively detailed description of a device of the type mentioned is contained, for example, in the “Amendments to UNECE regulations/regulation Number 83/Appendix 5” (United Nations, Economical and Social Council, Economic Commission for Europe; January 2008). Further, some devices are also already available commercially that contain these suggested elements and thus make corresponding measurements possible.
As is known, exhaust gases of combustion engines and particularly those of diesel engines contain not only classic aerosols (within the meaning of volatile suspended particles), but a mixture of solid and volatile suspended particles in a carrier gas, whereby the harmfulness of the corresponding analyses of the exhaust gases is almost exclusively caused by the solid particles. Devices of the type mentioned at the beginning that are of interest here therefore aim to eliminate the volatile particles that are not relevant for the definition of harmfulness prior to the final measurement, for which purpose the sample flow is sequentially diluted, heated and diluted again. In the pre-diluter that can, for example, be designed as per AT 9.603 U, the concentration of solid particles as well as of volatile aerosols is decreased in the sample flow. In the downstream heated evaporator, the volatile substances are transformed into the steam phase, whereby as a result of adjusting a corresponding pre-dilution, the concentration of volatile aerosols can be reduced to the extent that the steam pressure of these substances subsequent to the evaporator is so low that they no longer condensate even during subsequent cooling, as a result of which the sample stream that is to be cooled again subsequently then only still contains the solid particles that are to be counted.
To make the measurement or counting of the solid particles that are of interest possible in a wide range of the actual particle concentration in the particle-loaded gas stream, particularly the interplay of the dilution rate in the pre-diluter and secondary diluter, as well as the various states of heating and cooling of the sample flow must be able to be coordinated precisely or be controllable, which makes the specific executions of commercial systems that correspond to the recommendations of the UNECE regulations discussed at the beginning very complex, as a result of which they, for example, are scarcely used for test stands of local motor vehicle departments, auto repair shops and the like.
It is the problem of the present invention to design or improve a device of the type described at the beginning so that for a wide range of measurements a simple adjustment and control becomes possible.
According to the present invention, this problem is solved by a device of the type described at the beginning thereby, that the secondary dilutor is designed as a porous tube diluter and mounted between an outlet of the evaporator and a stabilization chamber from which the sample flow is diverted to the particle counter. A porous tube diluter of this type is provided with—at least on a part of the sample line running through it—circumferential openings, which flow from the outside, subject to at least slight over pressure, supplying substantially particle-free diluting gas to the sample line and can mix there with the sample flow in a manner that can be adjusted or controlled easily. As a result, the desired cooling of the sample flow also takes place directly, whereby it is ensured in the manner described previously that the volatile aerosol particles that evaporated during the prior heating cannot condensate again. As the result of the controllable dilution of the secondary diluter, together with the controllable dilution of the pre-diluter and the heating temperatures or cooling temperatures of the sample flow, a very wide range of the solid particles that are of interest can be covered in the final analysis in the concentrations that are to be measured in the downstream particle counter.
In a preferred embodiment of the invention, the secondary diluter is connected with the heated evaporator by a thermally isolated sample line, which ensures that heat exchange from the sample gas stream to the tube wall is prevented, and thus thermophoretic deposits of the aerosol particles can be avoided.
The secondary diluter that is designed as a porous tube diluter is preferably provided with a large number, preferably >1,000 circumferential openings in the sample line, preferably having a diameter of <0.15 mm, to which the additional diluting gas can be supplied as preferably a thermally controllable cold dilution stream from a sealed casing space that surrounds the sample line in this section in a sealed manner. As a result, a controlled secondary dilution and cooling of the sample flow takes place that is controlled extending over a relatively large circumferential section and very many mixing points.
In a further embodiment of the invention, the circumferential openings of the secondary diluter can also be designed tapered with a cross section that decreases from the outside toward the inside and further, also in the radial and/or axial direction deviating from the normal, aligned with the respective line surface, which results in further advantageous possibilities of influencing the mixture and thus dilution and cooling.
In a preferred embodiment of the invention, the stabilization chamber that is downstream of the secondary diluter is designed with a significantly larger cross section with respect to the sample line in the secondary diluter and has, in addition to an exhaust opening, at least one sampling connection for the particle counter. As a result of the thus present significant change of the Reynolds number at the transition into the stabilization chamber, a homogeneous mixture is formed with the diluting gas supplied in the secondary diluter, whereby representative samples can then be removed from the stabilization chamber, which are measured in the particle counter (for example, a known condensation counter) for the determination of the concentration of the remaining solid particles.
In the following, the invention is explained in more detail with the aid of the examples of embodiments that are illustrated schematically in the drawings.